1. Field of the Invention
The present invention relates to operation of two-side non-volatile memory devices. More particularly, the present invention relates to a method of determining an optimal reading voltage for a specific threshold voltage, or an optimal combination of threshold voltage and reading voltage, that can maximize the total current window in reading. The present invention also relates to programming and reading methods of two-side non-volatile memory, which are based on the optimal reading voltage or the optimal combination of threshold voltage and reading voltage being determined.
2. Description of the Related Art
A two-side non-volatile memory cell, such as, a nitride read-only memory (NROM) cell, typically includes a storage layer and a gate stacked on a semiconductor substrate and source/drain regions in the substrate beside the gate. The storage layer has two storage sites at two sides thereof near the source/drain, wherein each site is programmed and read independently. In a two-side non-volatile memory storing one bit per side, a side is not programmed or is programmed to a threshold voltage (Vt). In a two-side non-volatile memory storing two or more bits per side, a side is not programmed, or is programmed to Vt or one of the intermediate states between the non-programmed state and the Vt state.
Currently, a two-side non-volatile memory is usually read in reverse direction. For example, when the right side of a memory cell is previously programmed with channel hot electron injection (CHEI) using a current from right to left, the reading current is from left to right. The difference between the reading current at the non-programmed state and that at the Vt state is called a total current window, which varies with the reading voltage applied to the gate. When each side of a memory cell stores two or more bits, the total current window is divided by several intermediate states.
To improve the reading accuracy, the total current window is preferably as wide as possible so that each storage state can be clearly distinguished from the others. Conventionally, the reading voltage is set as or beneath the threshold voltage, but the resulting total current window is often insufficient. Meanwhile, there is no method to determine the optimal reading voltage, which maximizes the total current window, for reading a two-side non-volatile memory programmed with a specific threshold voltage.